The present invention relates to housings for holding large area infrared focal plane arrays. Such housings are commonly called dewars.
Two of the important criteria for a housing for a focal plane array are to maintain a vacuum around the array, and to keep the array at an extremely low temperature, such as 70-77 kelvin. Because focal planes are typically used in a variety of environments, the vacuum must be maintained for an extended time, such as five to ten years. Therefore, any seals on the structure must be impervious to molecular-sized or larger particles. A common technique for maintaining a low temperature for the focal plane array is to install it over a cryostat.
The structure must be rigid to hold the focal plane array in precise position. Additionally, the structure must be able to withstand the stresses of launch.
In the past, the dewar assemblies have been made of glass because of its high structural rigidity and low thermal conductivity for maintaining the cryostat. Such structures include a tubular member for receiving the cryostat, at one end of which is the focal plane array. A tubular cover covers the focal plane array and the cryostat tube. A window in one end of the cover allows the focal plane array during use to "see" the target. The other end of the cover is sealed to the end of the cryostat tube. The electrical connections to the focal plane array are routed along the length of the cryostat tubular member, inside the cover, and exit through the cover at the base of the structure. Each electrical connection through the cover must be sealed to preserve the vacuum over the focal plane array.
Although rigid, the glass is fragile, and thus is subject to breakage during manufacture, and during launch. In addition, providing the electrical connections between the focal plane array and the external circuitry requires sealing the wire connections through the glass. Such seals between metal and glass are extremely difficult to achieve, and are prone to cracking. Additionally, because of the different coefficients of expansion for the metal and glass, the seals are difficult to maintain during the extreme environmental temperature changes that may take place during manufacturing, testing, launching, and deploying the system in which the structure is installed. Furthermore, the proximity of the wires positioned in parallel along the length of the cryostat tube may give rise to interference between signals on adjacent wires.
Because of a trend toward the use of larger arrays, such as square arrays having 128 or 256 elements to a side, dewars must have an internal volume sufficient to accommodate such large arrays.